Cellular telephones are small, lightweight telephone units which do not require a hardwired connection with a telephone switching station. Instead, cellular telephones communicate with a network of transmitting/receiving stations via radio-frequency transmissions, allowing them to be used in any location that is in close enough proximity to the nearest receiving/transmitting station that the telephone can receive signals from that station and transmit signals to that station. All telephones within a certain area around a transmitting/receiving station will communicate with that station, and this area is called a "cell". If a cellular telephone is moving while in use, such as when it is being used in an automobile, the telephone may cross the boundary between one cell and another. In this case, communication between the telephone and the new cell's transmitting/receiving station is automatically established, and the transmitting/receiving station of the old cell is no longer involved in the communication.
Recent advances in electronics and increases in the number of transmitting/receiving station cells have led to a large increase in cellular telephone usage. A 1984 market study indicated that at that time, 94,000 cellular telephones were in operation. A 1987 study estimated that the number of cellular telephone users had grown to more than one million, and by 1990 there were an estimated four million telephone subscribers. More recent market research projects that within the next ten years, the cellular telephone market will reach thirty-six million subscribers.
One known problem with cellular telephones is that of poor sound and signal transmission quality as compared, for example, with convnetional telephones. Since cellular telephones communicate via radio-frequency, they are susceptible to the same sorts of interference and signal degradation phenomena as are encountered with ordinary television and radio transmissions. In addition, the noisy environment of an automobile, where cellular telephones are frequently used, further tends to degrade the sound quality for the user.
Another known difficultly with cellular telephones, especially in the automobile setting, is that the user must be able to operate and hold the telephone while driving. This makes the cellular telephone a potentially serious traffic hazard, since the driver must direct his attention to the road and the telephone at the same time, and may have to use only one hand for driving, thereby reducing his maneuverability. The user may keep his hands free for driving by holding the telephone between his head and shoulder, but this is an awkward position which similarly reduces the driver's maneuverability and also tends to be uncomfortable. One common solution to this problem is to provide the cellular telephone with "speaker-phone" capability, so that the user need not hold to phone while carrying on a phone conversation. However, using a speaker-phone in the automobile environment greatly aggravates the aforementioned problems of signal quality, since a speaker-phone microphone is necessarily less directional than a telephone handset microphone, and therefore normally much more sensitive to extraneous environmental noises, such as passing trucks, traffic, engine drone, and wind. Another drawback of the speaker-phone solution is that the user has no privacy during his telephone conversation; other passengers in the automobile, for example, can hear both parties in the telephone conversation.